In the field of power conversion technology, increasing the work duty and reducing the voltage stress are the most important demands. Basically, tertiary winding reset technology is desirable as it allows forward converter to transfer the power from input to output. However, the work duty of tertiary winding reset forward converter is limited at 50% maximum due to the requirement of voltage-second balance principle and the peak voltage of primary switching device is kept in two times of input voltage. In order to overcome limitations in work duty and voltage stress, the prior art has been devised the active clamp technology and passive clamped technology.
FIG. 1a shows the active clamp forward converter. The active clamp forward converter has been derived from conventional forward converter by adding the switch S.sub.A to select connecting the transformer to a reset capacitor C.sub.R when the primary power switch S.sub.P is turned OFF. The gate driving signals are separated by dead time with small period and supplied from a two channel complementary pulse-width-modulated controller. FIG. 1b shows the timing diagram of the circuit of FIG. 1a.
Active clamp reset technology reduces the voltage stress and allows the work duty is greater then 50% that the power handling of transformer is improved and reduce the power rating of primary switching device. However, the optimal gate drive waveform is generated from highly circuit complexity controller to prevent the damage from cross conduction between S.sub.A and S.sub.P, the high side driver is required to control the auxiliary switch S.sub.A. The voltage rating of auxiliary switch is equal to input voltage that the component selection is difficult to meet cost effect requirement in high input voltage applications.
FIG. 2a shows the passive clamp forward converter. The passive clamp forward converter has been derived from active clamp forward converter by relocating the clamp circuit from primary side to secondary side. Auxiliary switch S.sub.A is drives by secondary voltage to select connecting the transformer to a reset capacitor C.sub.R when the primary power switch S.sub.P is turned off. The gate drive signal is supplied from a single channel pulse-width-modulated controller. FIG. 2b shows the timing diagram of the circuit of FIG. 2a. This is a forward converter using the passive clamp technology and drive by a simple PWM controller. The current waveform of auxiliary switch shows low level magnetizing current with small period load current.
Passive clamp technology reduce the voltage rating of auxiliary switch and the circuit complexity of controller, it take advantages as active clamp technology but energy recover of leakage inductance. However, the maximum output current rated auxiliary switch is required to prevent the damage from simultaneous high current due to the transition of secondary voltage. The component selection is difficult to meet cost effect requirement in high output current applications.
What is the best reset technology? Passive drive, less circuit complexity of controller, low rating requirement in high frequency operation are the most important demands of transformer reset technology.